Materials that are resistant to heat at 150° C. or higher are demanded for covering or encapsulating semiconductor elements to form semiconductor devices of high heat resistance and high breakdown voltage. Among them, materials (encapsulants) for encapsulating optical materials such as optical semiconductor elements are demanded to excel also in properties such as transparency and flexibility, in addition to the heat resistance. Silicon-containing materials such as phenylsilicones are currently more and more used as encapsulants typically in backlight units of liquid crystal displays.
Patent Literature (PTL) 1 discloses a resin composition for optical element encapsulation, where the resin composition excels in transparency, ultraviolet resistance, and thermal coloration resistance. The resin composition contains, as a resin component, at least one silsesquioxane selected from the group consisting of liquid silsesquioxanes having a cage-like structure, containing an aliphatic carbon-carbon unsaturated bond, and being devoid of H—Si bonds; and liquid silsesquioxanes having a cage-like structure, containing a H—Si bond, and being devoid of aliphatic carbon-carbon unsaturated bonds. Unfortunately, however, the resin composition containing such a cage-like silsesquioxane gives a cured product that is relatively hard, has poor flexibility, and is susceptible to cracking or fracture.
PTL 2 discloses a curable composition that contains, as essential components, triallyl isocyanurate or another organic compound containing at least two carbon-carbon double bonds per molecule, where the carbon-carbon double bonds are reactive with a SiH group; a compound containing at least two SiH groups per molecule, such as a chain and/or cyclic polyorganosiloxane; and a hydrosilylation catalyst. These materials, however, are still insufficient in properties such as cracking resistance.